Flat panel displays (FPD) are products in the photoelectric industry, which combine the techniques of semiconductors, optics and chromatics. A trend is becoming increasingly recognizable in that FPD is gradually taking the place of the traditional cathode ray tubes (CRT). Among various flat panel displays, liquid crystal displays (LCD) have assumed a leading position, because of their light weight, thinness and capability of becoming a full-color display. Color filters are the key elements to render glistening and vivid pictures.
A color filter comprises three main components: a black-hued matrix, a color filter layer and an overcoat. Currently, commercial methods for manufacturing color filters include:
(1) dyeing, PA1 (2) etching, PA1 (3) pigment dispersion, PA1 (4) electrodeposition, and PA1 (5) printing. PA1 (1) The method combines the techniques of electrodeposition and lithography. Therefore, high precision patterns can be obtained, better than that obtainable from the electrodeposition coating method; PA1 (2) The pattern figure has a high degree of freedom, and both stripe and non-stripe patterns can be provided; and PA1 (3) Because it utilizes the advantageous characteristics of an electrodeposition process, the coated films exhibit uniform film thickness and excellent smoothness.
The dyeing method and the etching method primarily utilize dyes as the essential filtering materials. The advantages of using dyes as the essential filtering materials lie in their variant species, homogeneous chroma, high dyeability, high color intensity and high light transmissibility. Suitable dyes are disclosed in U.S. Pat. Nos. 4,820,619 and 4,837,098. Because of the relatively inadequate light and heat resistance of the dyeing materials, the methods of dyeing and etching have been largely replaced by the pigment dispersion method and the electrodeposition method that use pigments as the essential filtering materials. Pigments have superior light and heat resistance. One simply has to utilize a general pigment dispersion technique to control the particle size of the pigment to be less than 0.1 .mu.m, these two methods will enable pigments to perform color intensity and light transmissibility close to or even the same as dyes perform. Due to the above, the pigment dispersion method and the electrodeposition method have become the major methods on which industries rely in the manufacture of color filters.
Pigment dispersion methods, such as those disclosed in U.S. Pat. Nos. 5,085,973 and 4,786,148 and Japan Laid-Open Patent Publication No. 60-129739, involve the use of a photosensitive resin well dispersed in pigments and a photolithography technique to achieve a high resolution and a flexibility of pattern design. This method is currently the major manufacturing technique. However, due to the factors that (1) the efficacy of the materials is low (1%.about.2%), (2) the trend of applying to large sizes corresponding glass substrates is low and (3) the chances of using an expensive precisely aligning machine are quite frequent, the cost of production for such a method fails to comply with the trends of large sizes of color liquid displays and of lower prices.
Electrodeposition coating processes, such as that disclosed in U.S. Pat. No. 4,812,387, use an electrophoresis technique to electrodeposite an electrodeposition resin and a pigment which are both well dispersed in water onto a patterned transparent electrode substrate. A filter layer of a uniform thickness and of a good smoothness is obtained. The electrodeposition coating technique is limited in its applications. Owing to the design of the electrodes, electrodeposition coating process can only use a substrate with a stripe pattern of conductive film for implementation. Thus, it is impossible to arrange pixels freely.
Among all the processes for manufacturing color filters, the printing process is the least expensive process. However, it suffers from the problems of poor dimensional precision, smoothness and reliability. Printing processes are not well accepted by industries for making high quality electronic products, but are generally used in the manufacture of low-end products.
To address the problems and at the same time to preserve the advantages of pigment dispersion and electrodeposition coating process, Nippon Oil Company proposed an electrodeposition lithographic method (ED-litho) for making color filters which combined the electrodeposition (ED) coating method and the lithographic (litho) technique. As disclosed in U.S. Pat. Nos. 5,214,541 and 5,214,542, the contents of which are incorporated herein by reference, Nippon Oil Company discloses foremost an electrodeposition lithographic method. Said method involves the steps of exposing a photoresist layer on a transparent electrically conductive layer under a photomask having patterns of more than three different degrees of light transmittances for one time to form regions of different degrees of exposure energy, using different developer solutions to remove the photoresist layer stepwise and electrodepositing progressively the red, green and blue colors onto the exposed electrically conductive substrate. The electrodeposition lithographic method discussed above has several advantages:
However, the electrodeposition lithographic method requires developer solutions of at least three different levels of concentrations so as to selectively remove the exposed photoresist at different stages of the development process and to electrodeposite the colors of red, green and blue (R, G, B) thereunto, thus it allows only a relatively narrow process window within which tolerance is acceptable. Moreover, it is known to use basic aqueous developer solutions for positive photoresist. Under such circumstances, there exist only very limited options in selecting an appropriate electrodeposition resin. Additionally, there still exists photoresist on the substrate before the electrodeposition of all desired colors is accomplished. Thus, a culing (hardened) procedure at elevated temperature is impossible. In the examples of this reference, a color electrodeposition coating comprising an anionic electrodeposition resin is used. The acid value of said resin is in the range of from 100 to 500 mg KOH/g. Such type of anionic electrodeposition resin is easily influenced by developer solutions. Therefore, developer solutions of higher concentrations can not be applied. This results in a narrow tolerance of developer solutions. Although cationic electrodeposition resins have better basic resistance, they show the disadvantages of be easily yellowed and having a lower transmission. During the electrodepositing process, such type of resin tends to reduce the indium tin oxide (ITO), which is a commonly used transparent electrically conductive material of the transparent electrically conductive substrate, to black spots. The above recited technical limits are believed to be the main reasons why there are no commercialized products produced from the process.
Another method for making color filters which combined a electrodeposition (ED) coating method and a lithographic (litho) technique is disclosed in U.S. Pat. No. 5,641,595. The contents of said patent are incorporated herein by reference. Said method is characterized by utilizing the energy accumulate characteristic of positive photoresist in combination with light-curable electrodeposition resins. Said process involves the steps of coating a layer of positive photoresist onto a transparent electrically conductive substrate and exposing the positive photoresist layer to form regions of different initial levels of exposure energy. One of the regions reaches the full exposure energy of the positive photoresist. After a developing step, the photoresist on this region is removed and the corresponding electrically conductive substrate is uncovered. Said region is then electrodeposited to form the desired colors. When all steps of the method are accomplished, the substrate is subjected to an exposing step without alignment. The pixels electrodeposited previously are then cured by light. This step can avoid the electrodeposited color from being attacked by the developer solution used in the next stage. The regions which have not accumulated sufficient amounts of energy are subject to next exposure to ensure that the energy of the second region reaches the full exposure energy of said positive photoresist. After that, each region is developed with developer solution and electrodeposited with the desired color. Repeat the above steps until the arrangement of all the pixels is accomplished.
This energy incremental process possesses the function of developing the regions of different levels of exposure energy progressively. Because the method combines the advantages of using the photocurable anionic electrodeposited resins, making up the exposure energy to allow each region to reach the full exposure energy of the positive photoresist, and curing the film formed by the electrodeposition coating, the influence of the basic developer solution subsequently used on the electrodeposited pixels is eliminated and the developing step is simplified. However, the photocurable electrodeposited resins require a sufficient amount of exposure energy to cure the electrodeposited coating so as to defend against the attack of developer solutions. In order to possess a filtering function, pigment particles are dispersed into the electrodeposited coating. Thus, the energy need to expose the coating becomes even greater. This narrows the exposure tolerance of the photoresist. Moreover, the addition of photosensitive groups in the electrodeposited coating enhances the difficulty to achieve well dispersion and stability, and adversely influences the yield rate of the products.
The present invention intends to overcome the problems and to preserve the advantages of pigment dispersion and electrodeposition coating process for manufacturing color filters. The invention develops an excellent technique for manufacturing color filters by using a color electrodeposition coating containing an anionic electrodeposition resin having a low acid value in combination with a weak basic developed positive photoresist. Since the present invention utilizes an anionic electrodeposition resin having a low acid value in combination with a weak basic developed positive photoresist solution, the pixels of the corresponding regions electrodeposited previously can be baked at a normal drying temperature so as to defend against the attack of developer solutions used subsequently for developing other desired colors of pixels without influencing the functions of the photoresists. The method of the invention shows the advantages of having a high degree of freedom in pattern figures and a wide process window. Moreover, the manufacture color filters of large surface and a perfect yield rate of products are possible.